Tunable optical dispersion compensators

ABSTRACT

A tunable optical dispersion compensator includes an optical input port, an input variable optical coupler, at least two optical dispersion paths, an output variable optical coupler, and an optical output port. The input variable optical coupler is coupled to selectively split portions of the optical signal received from the optical input port into each of its output ports. The optical dispersion paths are each coupled to one of the output ports of the input variable optical coupler to impart dispersion compensation to each of the split portions of the optical signal. An output variable optical coupler selectively combines the split portions of the optical signal received on its input ports from the optical dispersion paths. The optical output port is coupled to the output variable optical coupler to output a dispersion compensated optical signal.

TECHNICAL FIELD

This disclosure relates generally to dispersion compensation, and inparticular but not exclusively, relates to integrated tunable opticaldispersion compensators.

BACKGROUND INFORMATION

Chromatic or optical dispersion is a phenomenon that causes theseparation of an optical wave into spectral components with differentwavelengths, due to a dependence of the wave's speed on its wavelength.When an optical signal or pulse is launched into an opticalcommunication channel (e.g., optic fiber), its envelope propagates at agroup velocity along the communication channel. Since this pulseincludes a range of spectral components, each spectral component travelsat a slightly different group-velocity, resulting in group-velocitydispersion (“GVD”), intramodal dispersion, or simply fiber dispersion.This separation phenomenon is also commonly referred to as pulsebroadening.

As the pulse travels down the optic fiber the spectral componentscontinue to spatially and temporally separate until the pulse is sobroad that the difference between a ‘0’ bit and a ‘1’ bit isindistinguishable on the receiving end. As the demand for greaterbandwidth increases, the temporal spacing between adjacent bitscontinues to shrink. If the travel distance is sufficiently large, theleading edge of a pulse can spatially overlap with the trailing edge ofa preceding pulse, causing the bits to blur into each other.

With wavelength-division multiplexing (“WDM”) communication systems,chromatic dispersion can be particularly troublesome since thistechnology multiplexes multiple optical carrier signals, each having adifferent wavelength, on a single fiber. Since each channel orwavelength is subject to a different amount of dispersion, dispersioncompensation techniques must be wavelength dependent.

As optical communication links are upgraded to higher speeds (e.g.,metro links upgraded to 10 Gbits/s from 2.5 Gbits/s, new 40 Gbits/slinks, etc.) dispersion is becoming the primary technical limitation.Current solutions use dispersion compensated fiber; however, thissolution requires vast lengths of fiber, suffers from substantialoptical loss, and is not tunable to meet the needs of WDM systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a functional block diagram illustrating a bulk dispersioncompensator, in accordance with an embodiment of the invention.

FIG. 2 is a line graph illustrating group delay versus wavelength forvarious splitting ratios of a bulk dispersion compensator, in accordancewith an embodiment of the invention.

FIG. 3 is a schematic diagram illustrating a dispersion compensator, inaccordance with an embodiment of the invention.

FIG. 4A is a line graph illustrating a transmission spectrum of aracetrack resonator, in accordance with an embodiment of the invention.

FIG. 4B is a line graph illustrating group delay imparted by a racetrackresonator, in accordance with an embodiment of the invention.

FIG. 5 is a cross-sectional view of a waveguide, in accordance with anembodiment of the invention.

FIG. 6 is a schematic diagram illustrating a splitter portion of avariable coupler and an evanescent mixing portion of the variablecoupler, in accordance with an embodiment of the invention.

FIG. 7A is a schematic diagram illustrating a racetrack resonatorcoupled to a bus waveguide via evanescent coupling, in accordance withan embodiment of the invention.

FIG. 7B is a schematic diagram illustrating a racetrack resonatorcoupled to a bus waveguide via dual evanescent couplers surrounding atunable Mach-Zehnder interferometer (“MZI”), in accordance with anembodiment of the invention.

FIG. 7C is a schematic diagram illustrating a racetrack resonatorcoupled to a bus waveguide via dual multi-mode interference couplerssurrounding a tunable MZI, in accordance with an embodiment of theinvention.

FIG. 8 is a functional block diagram illustrating a dispersion slopecompensator, in accordance with an embodiment of the invention.

FIG. 9 is a line graph illustrating dispersion compensation versuswavelength for two splitting ratios of a dispersion slope compensator,in accordance with an embodiment of the invention.

FIG. 10 is a schematic diagram illustrating a dispersion slopecompensator, in accordance with an embodiment of the invention.

FIG. 11 is a functional block diagram illustrating an opticalcommunication system implemented with a dispersion compensatorintegrated into a receiver, in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION

Embodiments of apparatuses, systems, and methods for tunable opticaldispersion compensation are described herein. In the followingdescription numerous specific details are set forth to provide athorough understanding of the embodiments. One skilled in the relevantart will recognize, however, that the techniques described herein can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring certain aspects.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

FIG. 1 is a functional block diagram illustrating a bulk dispersioncompensator 100, in accordance with an embodiment of the invention. Theillustrated embodiment of bulk dispersion compensator 100 includes aninput port 105, an input variable coupler 110, dispersion paths 115A and115B (collectively 115), an output variable coupler 120, and an outputport 125.

Bulk dispersion compensator 100 is a tunable dispersion compensator thatis capable of being tuned in real-time to compensate for a variety ofdifferent chromatic dispersions. In one embodiment, bulk dispersioncompensator 100 may be integrated onto an integrated optic platform(e.g. polymer, III-V semiconductor material, silica) or a semiconductorsubstrate (e.g., silicon substrate) and incorporated into an opticalreceiver (or repeater). In the illustrated embodiment, bulk dispersioncompensator 100 is configured to receive a single channel λ₁ inputsignal 130, to be tuned to impart selectable and appropriate dispersioncompensation to input signal 130, and to output a dispersion compensatedoutput signal 135. Bulk dispersion compensator 100 is well suited forintegration into individual customer site receivers of a metro network.

The components of bulk dispersion compensator 100 interoperate asfollows. Input signal 130 received at input port 105 is guided intoinput variable coupler 110. In the illustrated embodiment, inputvariable coupler 110 is a one-by-two variable splitter capable ofsplitting input signal 130 between its outputs 111 and 113 according toa selectable splitting ratio. Based on the selectable splitting ratio,input variable coupler 110 can either direct all optical power of inputsignal 130 to output 111, all optical power of input signal 130 intooutput 113, or direct a selectable portion into both outputs 111 and113.

From outputs 111 and 113, the split input signal 130 is guided alongdispersion paths 115A and 115B, respectively. Dispersion path 115Aimparts a dispersion compensation function D1 to the split portion ofinput signal 130 propagated along its branch, while dispersion path 115Bimparts a dispersion compensation function D2 to the split portion ofinput signal 130 propagated along its branch.

Dispersion paths 115A and 115B are coupled to inputs 121 and 123,respectively, of output variable coupler 120. Output variable coupler120 recombines the dispersion compensated split portions of input signal130 according to a selectable splitting ratio and outputs a dispersioncompensated output signal 135. During operation, to reduce optical loss,the splitting ratio of output variable coupler 120 should be tuned to besubstantially equivalent to the splitting ratio of input variablecoupler 110. For example, if 20% of the optical power of input signal130 is directed to dispersion path 115A and 80% of the optical power ofinput signal 130 is directed to dispersion path 115B, then the splittingratio of output variable coupler 120 should be set to a similar 20%:80%splitting ratio favoring dispersion path 115B.

In the extreme, if the splitting ratio of input variable coupler 110 isselected to direct all optical power of input signal 130 to dispersionpath 115A, then the overall dispersion compensation applied to outputsignal 135 will equal D1. In the opposite extreme, if the splittingratio of input variable coupler 110 is selected to direct all opticalpower of input signal 130 to dispersion path 115B, then the overalldispersion compensation applied will equal D2. By setting the splittingratio in between these extremes, the overall dispersion compensationapplied will range between D1 and D2.

In addition to selectively setting the overall dispersion compensationranging between D1 and D2, embodiments of the invention include theability to tune or select the individual dispersion compensationfunctions D1 and D2 of dispersion paths 115. Dispersion compensationfunctions D1 and D2 may be configured/preset in advance of operation ortuned during operation in real-time.

FIG. 2 is a line graph 200 illustrating group delay versus wavelengthfor various splitting ratios ‘R’ of bulk dispersion compensator 100, inaccordance with an embodiment of the invention. Dispersion is thegradient of group delay. Therefore, line 205, which illustrates groupdelay for a splitting ratio R=0 (e.g., all optical power of input signal130 directed to dispersion path 115B), has a negative slope and willimpart a negative dispersion compensation. Correspondingly, line 210,which illustrates group delay for a splitting ratio R=1 (e.g., alloptical power of input signal 130 directed to dispersion path 115A), hasa positive slope and will impart a positive dispersion compensation. Theother lines illustrate splitting ratios R ranging between 0 and 1 forimparting varying degrees of positive and negative dispersioncompensation. Accordingly, embodiments of the invention are capable ofselectively imparting both positive and negative dispersioncompensation.

FIG. 3 is a schematic diagram illustrating one possible implementationof bulk dispersion compensator 100, in accordance with an embodiment ofthe invention. However, it should be appreciated that other embodimentsmay be implemented with other substitute subcomponents. Furthermore, theillustrated embodiment is not necessarily drawn to scale, but rather ismerely intended for descriptive purposes.

The illustrated embodiment of input variable coupler 110 includes asymmetrical Mach-Zehnder interferometer (“MZI”) 305 having refractiveindex controllers 310A and 310B coupled to each of its arms 315A and315B, respectively. Refractive index controllers 310 may be implementedwith a variety of devices that affect the optical length of each arm 315(e.g., resistive thermal heaters, carrier injection via a pn junction,etc.). In one embodiment, only one arm 315 includes a refractive indexcontroller 310, while the other arm does not. MZI 305 is then coupled toeach dispersion path 115.

The illustrated embodiment of dispersion path 115A includes a buswaveguide 325A optically coupled to cascaded racetrack resonators 330A.Similarly, dispersion path 115B includes a bus waveguide 325B opticallycoupled to cascaded racetrack resonators 330B. Although FIG. 3illustrates each dispersion path 115 as including five racetrackresonators 330, embodiments of dispersion paths 115 may include more orless racetrack resonators.

Three primary factors affect the performance of racetrack resonators330—loss of each racetrack, bias of each racetrack, and couplingstrength of each racetrack. The loss of each racetrack resonator 330 isdetermined by its radius of curvature (too small and light will bleedout), material absorption, and scattering due to imperfections in thesidewalls and lattice structures of each racetrack resonator 330. In oneembodiment, racetrack resonators 330 have a radius of curvature R1 equalto approximately 200 μm. To bias each racetrack resonator 330, itsresonance condition may be altered by changing its optical length viathermal control, charge carrier injection, or otherwise. In oneembodiment, racetrack resonators 330 include refractive indexcontrollers to tune or bias the individual racetrack resonators 330 toresonate at different wavelengths. Finally, the magnitude of dispersionimparted by each racetrack resonator 330 may be manipulated bycontrolling the coupling strength of each racetrack resonator 330 to itcorresponding bus waveguide 325. In one embodiment, each racetrackresonator 330 includes a variable coupler to selectively tune itscoupling strength to its corresponding bus waveguides 325.

The illustrated embodiment of output variable coupler 120 also includesa symmetrical MZI 305 having refractive index controllers 310A and 310Bcoupled to each of its arms 315A and 315B. The inputs of each arm 315are coupled to a corresponding dispersion path 115 to receive thedispersion compensated split portions of input signal 130. Finally, theoutput of output variable coupler 120 combines the dispersioncompensated output signal 135 onto output port 125.

FIGS. 4A and 4B illustrate how each racetrack resonator 330 operates toimpart dispersion compensation, in accordance with an embodiment of theinvention. FIG. 4A is a line graph 405 illustrating a transmissionspectrum of a single racetrack resonator 330, while FIG. 4B is a linegraph 410 illustrating a group delay of input signal 130 propagatingthrough a single racetrack resonator 330.

As illustrated in FIG. 4A, the transmission spectrum of each racetrackresonator 330 resembles an inverted Gaussian, which harmonically repeatsitself. A transmission minimum resides at the trough of each invertedGaussian, with the separation distance between adjacent transmissionminima equaling the free spectral range (“FSR”) of the racetrackresonator 330. The transmission minimum can be translated towards longeror shorter wavelengths by changing the refractive index of the racetrackresonator (i.e., biasing the racetrack resonator).

The wavelength location of where the transmission minimum aligns on thegroup delay waveform illustrated in FIG. 4B, will determine themagnitude and sign (i.e., positive or negative) of the dispersioncompensation imparted by a particular racetrack resonator 330. Sincedispersion is the gradient of the group delay waveform, if thetransmission minimum is translated to the left of line 415, then theimparted dispersion compensation will be positive, since the slope ofthe group delay waveform is positive. If the transmission minimum istranslated to the right of line 415, then the imparted dispersioncompensation will be negative, since the slope of group delay waveformis negative.

Referring to FIGS. 3 and 4A, by biasing racetrack resonators 330A suchthat their transmission spectrums overlap, then a smooth dispersioncompensation function D1 can be achieved. For example, each racetrackresonator 330A could be biased such that its inverted Gaussian waveformoverlaps the inverted Gaussian waveform of at least one other racetrackresonator 330A. Racetrack resonators 330B could be biased in a similarmanner. When light propagates along bus waveguides 325, those spectralcomponents matching a resonance condition (i.e., resonance wavelength)of a particular racetrack resonator 330, will couple into the particularracetrack resonator 330 and become temporarily trapped. In this manner,racetrack resonators 330 can be tuned to affect specific spectralcomponents in a selectable manner. By cascading several racetrackresonators 330 each tuned to a specified resonance wavelength withoverlapping transmission spectrums, a smooth desired dispersioncompensation function can be achieved.

FIG. 5 is a cross-sectional view of waveguide arm 315A along line A-A′in FIG. 3, in accordance with an embodiment of the invention. Theillustrated embodiment of waveguide arm 315A is a rib waveguideincluding a rib section 505 and slab section 510. In one embodiment, allwaveguide sections of bulk dispersion compensator 100 include a ribwaveguide cross-section. In other embodiments, other waveguidecross-sectional geometries may be used. A demonstrative embodiment isfabricated using SiO₂ for material layers 515 and 520, Si for ribsection 505, and Si for slab section 510. In one demonstrativeembodiment, the physical dimensions are as follows: L1≅1.5 μm, W1≅0.75μm, W2≅1.5 μm, L2≅2.0 μm, W3≅0.5 μm.

FIG. 6 is a schematic diagram illustrating a splitter portion 605 and anevanescent mixing portion 610 of input variable coupler 110, inaccordance with an embodiment of the invention. As a reminder, FIG. 6 isnot drawn to scale. In a demonstrative embodiment of MZI 305, whererefractive index controllers 310 are resistive heaters, length L5≅1.0mm.

Splitter portion 605 may be implemented with a variety of opticalsplitting devices including a regular Y-splitter, an evanescent splitter(illustrated), a one-by-two multi-mode interference (“MMI”) splitter, orthe like. In one demonstrative embodiment, the separation width W4≅0.65μm and the interaction length L3≅750 μm. Other dimensions may be used.

Evanescent mixing portion 610 is implemented by bringing the splitportions of input signal 130 together for a fixed interaction length sothat the two waves can evanescently mix. By adjusting the opticallengths of the two waveguide arms 315A and 315B of MZI 305, a relativephase difference between the two split portions of input signal 130 isinduced. Based on this phase difference, the two split portions can beselectively guided either entirely into bus waveguide 325A, entirelyinto bus waveguide 325B, or partially into both bus waveguides 325A and325B. For example, if the relative phase difference is π/2, then alloptical energy is directed into bus waveguide 325A; however, if therelative phase difference is 3π/2, then all optical energy is directedinto bus waveguide 325B. In the demonstrative embodiment, the separationwidth W5≅0.8 μm and an interaction length L4≅230 μm. Other dimensionsmay be used.

FIG. 7A is a schematic diagram illustrating a racetrack resonator 705coupled to bus waveguide 325A via evanescent coupling, in accordancewith an embodiment of the invention. Racetrack resonator 705 is onepossible embodiment of racetrack resonators 330. The illustratedembodiment of racetrack resonator 705 includes an evanescent couplingsection 710 to couple optical signals to/from bus waveguide 325A. In onedemonstrative embodiment, evanescent coupling section 710 has aninteraction length L6≅490 μm and a separation distance W6≅0.8 μm,refractive index controller 715 has a length L7≅1.0 mm, and radiusR1≅200 μm. Other dimensions may be used.

FIG. 7B is a schematic diagram illustrating a racetrack resonator 720coupled to bus waveguide 325A via dual evanescent couplers 725surrounding a tunable MZI 730, in accordance with an embodiment of theinvention. Racetrack resonator 720 is one possible embodiment ofracetrack resonators 330. Tunable MZI 730 and evanescent couplers 725enable adjustment of the coupling strength between racetrack resonator720 and bus waveguide 325A. For example, adjustment of refractive indexcontroller 735 may allow for tunable coupling efficiencies ranging from5% to 95% between bus waveguide 325A and racetrack resonator 720.Although FIG. 7B illustrates MZI 730 as including only a singlerefractive index controller 735 on one arm, other embodiments mayinclude two refractive index controllers, one on each arm. In onedemonstrative embodiment, interaction lengths L8≅230 μm, separationdistance W6≅0.8 μm, length L7≅1.0 mm, and radius R1≅200 μm. Otherdimensions may be used.

FIG. 7C is a schematic diagram illustrating a racetrack resonator 740coupled to bus waveguide 325A via dual multi-mode interference (“MMI”)couplers 745 surrounding tunable MZI 730, in accordance with anembodiment of the invention. Racetrack resonator 740 is one possibleembodiment of racetrack resonators 330. Tunable MZI 730 and MMI couplers745 enable adjustment of the coupling strength between racetrackresonator 740 and bus waveguide 325A. MMI couplers 745 operate astwo-by-two optical couplers and tunable MZI 730 operates to adjust thesplitting ratio to guide a selectable amount of light into racetrackresonator 740. In one demonstrative embodiment, length L9≅458 μm, widthW7≅10 μm, and radius R1≅200 μm. Other dimensions may be used.

FIG. 8 is a functional block diagram illustrating a dispersion slopecompensator 800, in accordance with an embodiment of the invention. Theillustrated embodiment of dispersion slope compensator 800 includesinput port 105, an optical demultiplexer 810, an input variable coupler815, dispersion paths 115A and 115B, an output variable coupler 820, anoptical multiplexer 825, and output port 125.

Dispersion slope compensator 800 is a tunable dispersion compensatorthat is capable of being tuned in real-time to compensate for a varietyof different chromatic dispersions. In one embodiment, dispersion slopecompensator 800 may be integrated onto an integrated optic platform(e.g. polymer, III-V semiconductor material, silica) or integrated on asemiconductor substrate (e.g., silicon substrate) and incorporated intoan optical receiver (or repeater). Dispersion slope compensator 800 isconfigured to receive a multi-channel λ_(1toN) input signal 830, to betuned to impart selectable and appropriate slope dispersion compensationto input signal 830, and to output a dispersion compensated outputsignal 835.

In one embodiment, dispersion slope compensator 800 is well suited foruse with long haul optical carriers (e.g., transoceanic carriers) havingmany communication channels (e.g., 100) multiplexed on different carrierwavelengths. Dispersion slope compensator 800 imparts a tunable slopedispersion compensation function across all channels multiplexed oninput signal 830. This tunable slope dispersion compensation function issuitable to compensate for residual dispersion remaining after inputsignal 830 has traversed a length of dispersion compensated fiber. Sincedispersion compensated fiber imparts a fixed dispersion compensationfunction set to compensate for a single wavelength (usually the centercarrier wavelength), channels carried on shorter and longer carrierwavelengths generally will have some amount of residual dispersion.Dispersion slope compensator 800 can tune the slope of the dispersioncompensation function to correct the residual dispersion at the shortand long wavelengths, while leaving the center wavelength substantiallyundistorted.

The components of dispersion slope compensator 800 interoperate asfollows. Input signal 830 received at input port 105 is guided intooptical demultiplexer 810. Optical demultiplexer 810 separatesmulti-channel input signal 830 into at least two groups λ_(A) and λ_(B).λ_(A) includes a first portion of the channels or carrier wavelengths(e.g., short carrier wavelengths), while λ_(B) includes a second portionof the channels (e.g., long carrier wavelengths). Input variable coupler815 then selectively splits portions of each of these two groups λ_(A)and λ_(B) into dispersion paths 115 according to the tunable splittingratio. In the illustrated embodiment, input variable coupler 815 is atwo-by-two variable splitter capable of splitting optical power from thetwo groups λ_(A) and λ_(B) of input signal 830 between dispersion paths115, according to the selectable splitting ratio. Optical power fromgroup λ_(A) may be directed to one or both of dispersion paths 115.Similarly, optical power from group λ_(B) may be directed to one or bothof dispersion paths 115, as well. The split portions from each groupλ_(A) and λ_(B) are then propagated through dispersion paths 115 toimpart the dispersion compensation functions D1 and D2 thereto.

Output variable coupler 820 is coupled to both dispersion paths 115 andrecombines (mixes) optical power received from each dispersion path 115according to the selectable splitting ratio. Output variable coupler 820outputs the dispersion compensated groups λ_(A) and λ_(B) to opticalmultiplexer 825. Optical multiplexer 825 then multiplexes the two groupsλ_(A) and λ_(B) back together into a single dispersion compensatedoutput signal 835.

FIG. 9 is a line graph 900 illustrating dispersion compensation versuswavelength for two splitting ratios of dispersion slope compensator 800,in accordance with an embodiment of the invention. Line 905 illustratesthe imparted dispersion compensation when the splitting ratio R of inputand output variable couplers 815 and 825 is set to R=0. Setting thesplitting ratio R=0 imparts a positive dispersion compensation over theuseful bandwidth delineated by the shading. Line 910 illustrates theimparted dispersion compensation when the splitting ratio R of input andoutput variable couplers 815 and 825 is set to R=1. Setting thesplitting ratio R=1 imparts a negative dispersion compensation over theuseful bandwidth. By adjusting the splitting ratio R between these twoextreme values, the imparted dispersion compensation can be tuned inreal-time to compensate for a variety of residual dispersion profiles.

FIG. 10 is a schematic diagram illustrating one possible implementationof dispersion slope compensator 800, in accordance with an embodiment ofthe invention. However, it should be appreciated that other embodimentsmay be implemented with other substitute subcomponents. Furthermore, theillustrated embodiment is not necessarily drawn to scale, but rather ismerely intended for descriptive purposes.

The illustrated embodiment of optical demultiplexer 810 includes anasymmetrical MZI 1005. In one embodiment, the upper branch ofasymmetrical MZI 1005 is approximately 20 μm longer than the bottombranch. Asymmetrical MZI 1005 includes a splitting portion and mixingportion with similar dimensions as splitting portion 605 and evanescentmixing portion 610 of MZI 305 (see FIG. 6), respectively. In oneembodiment, optical multiplexer 830 is similar to optical demultiplexer810, but mirrored.

In the illustrated embodiment, input variable couple 815 and outputvariable coupler 825 include tunable MZIs 305 (see FIG. 6). Similarly,dispersion paths 115 are coupled and operate in a similar manner asdescribed above in connection with bulk dispersion compensator 100.

FIG. 11 is a functional block diagram illustrating an opticalcommunication system 1100 implemented with a dispersion compensatorintegrated into a receiver, in accordance with an embodiment of theinvention. The illustrated embodiment of optical communication system1100 includes a transmitter 1105, a communication channel 1110 (e.g.,optic fiber), a repeater amplifier 1115, and a receiver 1120. Theillustrated embodiment of receiver 1120 includes a dispersioncompensator 1125 (e.g., bulk dispersion compensator 100 or dispersionslope compensator 800), an optical-to-electrical (“O-E”) converter 1130(e.g., photo detector), and electronic circuitry 1135. In oneembodiment, dispersion compensator 1125 and O-E converter 1130 may beintegrated onto a single semiconductor die. In one embodiment, all thecomponents of receiver 1120 may all be integrated onto one semiconductordie. If communication channel 1110 is a customer link of metro network,then dispersion compensator 1125 may be implemented as bulk dispersioncompensator 100. However, if communication channel 1110 is a long haulcarrier link, then dispersion compensator 1125 may be implemented asdispersion slope compensator 800.

During operation, transmitter 1105 converts electrical data to opticaldata and launches the optical data onto communication channel 1110. Inone embodiment, the optical data is modulated onto one or more carrierwavelengths centered about the 1.55 μm wavelength. Repeater 1115receives the optical data, amplifies it, and then retransmits it alongcommunication channel 1110. While propagating along communicationchannel 1110, the optical data may degrade due to chromatic dispersion.When the optical data is received at receiver 1120, its dispersion iscompensated by dispersion compensator 1125, converted back to theelectrical realm by O-E converter 1130, and then manipulated byelectronic circuitry 1135. In one embodiment, dispersion compensator1125 includes control circuitry to automatically adapt, in real-time,its dispersion compensation to optimize the restoration of the receivedoptical data.

The illustrated embodiment of electronic circuitry 1135 includes one ormore processors 1140, system memory 1145, a data storage unit (“DSU”)1150, and non-volatile (“NV”) memory 1155. The elements of electroniccircuitry 1135 are interconnected as follows. Processor(s) 1140 iscommunicatively coupled to system memory 1145, DSU 1150, and NV memory1155 to send and to receive instructions or data thereto/therefrom. Inone embodiment, NV memory 1155 is a flash memory device. In otherembodiments, NV memory 1155 includes any one of read only memory(“ROM”), programmable ROM, erasable programmable ROM, electricallyerasable programmable ROM, or the like. In one embodiment, system memory1145 includes random access memory (“RAM”), such as dynamic RAM(“DRAM”), synchronous DRAM (“SDRAM”), double data rate SDRAM (“DDRSDRAM”), static RAM (“SRAM”), and the like. DSU 1150 represents anystorage device for software data, applications, and/or operatingsystems, but will most typically be a nonvolatile storage device. DSU1150 may optionally include one or more of an integrated driveelectronic (“IDE”) hard disk, an enhanced IDE (“EIDE”) hard disk, aredundant array of independent disks (“RAID”), a small computer systeminterface (“SCSI”) hard disk, and the like.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various modifications arepossible within the scope of the invention, as those skilled in therelevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification. Rather, the scope of the invention is tobe determined entirely by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

1. An apparatus, comprising: an input variable optical coupler includingoutput ports, the input variable optical coupler coupled to selectivelysplit portions of an optical signal received from an optical input portinto each of the output ports; optical dispersion paths each coupled toone of the output ports of the input variable optical coupler to impartdispersion compensation to each of the split portions of the opticalsignal, wherein each of the optical dispersion paths includes a buswaveguide and a plurality of cascaded racetrack resonators opticallycoupled along the bus waveguide; an output variable optical couplerincluding input ports each coupled to one of the optical dispersionpaths to selectively combine the split portions of the optical signal;and an optical output port coupled to the output variable opticalcoupler to output a dispersion compensated optical signal.
 2. Theapparatus of claim 1, wherein the optical dispersion paths comprisetunable optical dispersion paths capable of imparting a selectabledispersion compensation.
 3. The apparatus of claim 2, wherein theracetrack resonators have overlapping transmission spectrums.
 4. Theapparatus of claim 2, wherein the tunable optical dispersion paths eachfurther include a plurality of heaters each thermally coupled to one ofthe racetrack resonators to thermally adjust a transmission minimum ofthe corresponding racetrack resonator.
 5. The apparatus of claim 2,wherein the tunable optical dispersion paths each further include aplurality of carrier injection units each to inject charge carriers intoone of the racetrack resonators to adjust a transmission minimum of thecorresponding racetrack resonator.
 6. The apparatus of claim 2, whereinthe racetrack resonators are evanescently coupled to the bus waveguide.7. The apparatus of claim 2, wherein each of the racetrack resonatorsincludes: a Mach-Zehnder interferometer (“MZI”) disposed along the buswaveguide; first and second evanescent couplers disposed along the buswaveguide on either side of the MZI; and a refractive index controllerdisposed on at least one branch of the MZI, the refractive indexcontroller configured to selectively adjust an index of refraction ofthe at least one branch.
 8. The apparatus of claim 2, wherein each ofthe racetrack resonators includes: a Mach-Zehnder interferometer (“MZI”)disposed along the bus waveguide; first and second multi-modeinterference (“MMI”) modules disposed along the bus waveguide on eitherside of the MZI; and a refractive index controller disposed on at leastone branch of the MZI, the refractive index controller configured toselectively adjust an index of refraction of the at least one branch. 9.The apparatus of claim 1, wherein the input and output variable opticalcouplers each include: a Mach-Zehnder interferometer (“MZI”) havingsubstantially equal length branches; and refractive index controllerscoupled to each of the branches of the MZI to selectively adjust indexesof refraction of the branches.
 10. The apparatus of claim 1, wherein theapparatus comprises a bulk dispersion compensator, wherein the inputvariable optical coupler comprises a one-by-two tunable splitter, andwherein the output variable optical coupler comprises a two-by-onetunable combiner.
 11. The apparatus of claim 1, wherein the inputvariable optical coupler, the optical dispersion paths, and the outputvariable optical coupler are integrated on any one of a semiconductorsubstrate, a III-V semiconductor substrate, a silicon substrate, apolymer substrate, or a glass substrate.
 12. An apparatus comprising: aninput variable optical coupler including output ports, the inputvariable optical coupler coupled to selectively split portions of anoptical signal received from an optical input port into each of theoutput ports; optical dispersion paths each coupled to one of the outputports of the input variable optical coupler to impart dispersioncompensation to each of the split portions of the optical signal; anoutput variable optical coupler including input ports each coupled toone of the optical dispersion paths to selectively combine the splitportions of the optical signal; an optical output port coupled to theoutput variable optical coupler to output a dispersion compensatedoptical signal; an optical demultiplexer disposed between the opticalinput port and the input variable optical coupler; and an opticalmultiplexer disposed between the output variable optical coupler and theoptical output port.
 13. The apparatus of claim 12, wherein the inputand output variable optical couplers comprise two-by-two tunablecouplers.
 14. The apparatus of claim 13, wherein the opticaldemultiplexer and the optical multiplexer comprise asymmetricalMach-Zehnder interferometers.
 15. A method of dispersion compensation,comprising: splitting an optical signal into first and second portionsbased on a first tunable splitting ratio; applying a first dispersioncompensation function to the first portion by propagating the firstportion along a first dispersion path; applying a second dispersioncompensation function to the second portion by propagating the secondportion along a second dispersion path; recombining the first and secondportions based on a second tunable splitting ratio; outputting adispersion compensated version of the optical signal; and adjusting anoverall dispersion compensation between the first dispersioncompensation function and the second dispersion compensation function bychanging the first and second splitting ratios.
 16. The method of claim15, wherein the first tunable splitting ratio and the second tunablesplitting ratio are selected to be substantially equivalent.
 17. Themethod of claim 15, wherein the first dispersion compensation functionis represented by D1 and the second dispersion compensation function isrepresented by D2, and wherein D2=−D1.
 18. The method of claim 15,wherein applying the first and second dispersion compensation functionsto the first and second portions, respectively, comprises: propagatingthe first portion along a first bus waveguide optically coupled to afirst plurality of cascaded racetrack resonators; and propagating thesecond portion along a second bus waveguide optically coupled to asecond plurality of cascaded racetrack resonators.
 19. A method ofdispersion compensation, comprising: demultiplexing a multi-channeloptical signal into two input groups of channels; coupling a firstportion from each of the two input groups of channels into a firstdispersion path and a second portion from each of the two input groupsof channels into a second dispersion path, based on a first tunablecoupling ratio; applying a first dispersion compensation function to thefirst portion by propagating the first portion along the firstdispersion path; applying a second dispersion compensation function tothe second portion by propagating the second portion along the seconddispersion path; coupling the first and second portions from the firstand second dispersion paths into two output groups of channels, based ona second tunable coupling ratio; and multiplexing the two output groupsinto a dispersion compensated multi-channel optical signal.
 20. Themethod of claim 19, wherein the first tunable coupling ratio and thesecond tunable coupling ratio are selected to be substantiallyequivalent.
 21. The method of claim 19, wherein applying the firstdispersion compensation function to the first portion further comprisespropagating the first portion along a first bus waveguide coupled to afirst plurality of cascaded racetrack resonators, and wherein applyingthe second dispersion compensation function to the second portionfurther comprises propagating the second portion along a second buswaveguide coupled to a second plurality of cascaded racetrackresonators.
 22. The method of claim 21, further comprising: tuning thefirst dispersion compensation function by adjusting a first transmissionminimum of at least one of the first plurality of racetrack resonators;and tuning the second dispersion compensation function by adjusting asecond transmission minimum of at least one of the second plurality ofracetrack resonators.
 23. The method of claim 19, further comprising:adjusting an overall dispersion compensation applied to themulti-channel optical signal by changing the first and second splittingratios.
 24. An optical receiver, comprising: a processor coupled tosynchronous dynamic random access memory (“SDRAM”), the processor tostore and manipulate electrical data in the SDRAM; anoptical-to-electrical (“O-E”) converter coupled to the processor toconvert optical data to the electrical data; and a dispersioncompensation unit coupled to the O-E converter, the dispersioncompensation unit including: an input variable optical coupler includingoutput ports, the input variable optical coupler coupled to selectivelysplit portions of the optical data into each of the output ports;optical dispersion paths each coupled to one of the output ports of theinput variable optical coupler to impart dispersion compensation to eachof the split portions of the optical data, wherein each of the opticaldispersion paths includes a bus waveguide and a plurality of cascadedracetrack resonators optically coupled along the bus waveguide; and anoutput variable optical coupler including input ports each coupled toone of the optical dispersion paths to selectively combine the splitportions of the optical data.
 25. The optical receiver of claim 24,wherein the dispersion compensation unit comprises a bulk dispersioncompensator, wherein the input variable optical coupler comprises aone-by-two tunable splitter, and wherein the output variable opticalcoupler comprises a two-by-one tunable combiner.
 26. The opticalreceiver of claim 24, wherein the dispersion compensation unit comprisesa dispersion slope compensator, the dispersion slope compensator furthercomprising: an optical demultiplexer disposed between the optical inputport and the input variable optical coupler; and an optical multiplexerdisposed between the output variable optical coupler and the opticaloutput port.
 27. The optical receiver of claim 24, wherein the inputvariable optical coupler, the optical dispersion paths, and the outputvariable optical coupler are integrated on any one of a semiconductorsubstrate, a III-V semiconductor substrate, a silicon substrate, apolymer substrate, or a glass substrate.